1. Field of the Invention
The present invention relates to a gas generating device for inflating an air bag to protect a driver in a vehicle, and more particularly, to a gas generating device equipped with filters.
2. Description of the Related Art
A conventional gas generating device of this type is disclosed, for example, in Japanese Patent Publication No. 2-19020. This gas generating device 30 retains a pelletized gas generating composition 36 in its combustion chamber 31, as shown in FIG. 8. In the combustion chamber 31 are placed an igniting device for igniting the gas generating composition 36, and a primary filter 32 for collecting the residue of a burnt gas generating composition. Provided around the combustion chamber 31 is a cooling chamber 37 where a secondary filter 39 is located to cool gas and to condense solid state particles (burning residue). When the igniting device is activated, the gas generating composition 36 burns to generate gas, which passes through the primary filter 32 and enters a diffusion space. The gas is then purified by the secondary filter 39 so as to be supplied through a gas exhaust port into an air bag.
The primary filter 32 consists of three elements: a layered metal wide mesh screen 33 in the vicinity of the inner wall of the combustion chamber 31, a glass fiber cloth 34 located inward of the screen 33, and a layered metal fine mesh screen 35 located inward of the cloth 34.
The conventional gas generating device uses a glass fiber cloth for the primary filter 32. While the glass fiber cloth effectively collects burning residue, it may be melted by the heat of the gas generating composition. When melted, the glass fiber cloth becomes sticky. The glass fiber and the residue will therefore be integrated so as to cause clogging in the glass fiber cloth 34, or the melted glass fiber will be adhered to the screen 33 to cause clogging. Under these circumstances, the inner pressure of the combustion chamber 31 substantially rises, and the walls of the combustion chamber 31 and the cooling chamber 37 are deformed, so that the gas is discharged without passing through the secondary filter 39, i.e., a short path phenomenon occurs.
The secondary filter 39 of the conventional gas generating device, like the primary filter 32 consists of three elements. These elements are a multi-layered wide metal wire mesh, a multi-layered aluminum silicate cover that surrounds the wire mesh, and a fine screen. These are arranged outward in the named order.
The individual elements of the secondary filter 39 tend to have inappropriately high bulk densities. Thus, the gases do not pass therethrough a quickly as desired. Thus the internal pressure in the gas generating device may rise abnormally rise or clogging may occur in the filter.
Japanese Unexamined Patent Publication No. 2-155861 discloses the use of a heat-resistant porous material, an inorganic fibrous sheet, a porous ceramic filter, a dutch woven wire mesh and a fine wire mesh in combination, as a secondary filter. The dutch woven wire mesh is woven like the surface of tatami (Japanese mat) with vertical lines Lv and transverse lines Lt which extend close to one another in a transverse direction, as shown in FIG. 9. Since the bulk densities of the elements of such a secondary filter as those of the above-described secondary filter, are improperly set, it is not possible to solve the problem about the normal rise of the internal pressure of the gas generating device and the clogging of the filters.